[mapguide-users] HQ aerial photography with MGOS - howto

[Krunoslav]

First of all, sorry on my bad English, I hope that next article will be
understandable enough. Second, special thanks to Frank Warmerdam, who helped
me a lot with handling hq tiff images.

I wrote this small howto, even for reminding myself, if I forgot how I did
it :)

This howto is based on my experience with Mapguide Opensource, high quality
aerial photography and FDO provider for GDAL. Images I used were GeoTIFF
images with resolution 1px=0,5m.

If you have large number of high-quality georeferenced images (ortophoto or
other aerial images), obvious problems are:

-	size of images (usually an area of 3x2km high-quality 24bit RGB image has
over 100MB)
-	sacrificing quality of images to improve speed
-	large number of files

So, our goals are: 

-	to maintain highest quality of images (no compression) to show full
details at closer zoom levels
-	to optimize loading time of images at higher zoom levels

Our basic approach should be to optimize images, and to reduce number of
files as possible. So, to reduce number of image files, we must merge as
many tiff as possible (please keep in mind that geotiff image size can’t be
more than 4GB). So we must aim for the images close to 4GB. For merging, the
best tool I’ve found is FWTools (http://home.gdal.org/fwtools). 
Since GDAL provider for raster doesn’t support transparency, please keep in
mind to merge images so that they close a rectangle area. Otherwise, you
will have black background where there are no images.  Now, let’s merge
images: run Fwtools shell, and enter:

cd bin
gdal_merge.bat image1.tif image2.tif image3.tif –o out.tif

This will merge images 1,2 and 3 to out.tif. Now we have to optimize
out.tif: we will convert this image to tiled format, and to add overviews:

gdal_translate -co TILED=YES out.tif out_tiled.tif 
gdaladdo out_tiled.tif 2 4 8 16 32 64 128

This image is now optimized. Images should load really fast at lower zoom
levels.

Anyway, if you have high-res images as I do (1px=0,5meters), performances
will still be poor at further zoom levels. This is understandable, since
Provider for raster has to read large image area, and enormous file size.
But, we can use another method here:

Resize this high-quality tiff’s for usage above closest zoom levels. For
example, I use 1px=2meters for zoom levels 1:2000-1:10000, 1px=5meters
1:50000 and 1px=10meters for levels above 1:50000. This way you will have
few small tiff images, which are still good enough for those zoom levels,
because at 1:50000 you really can’t see any difference between 0,5 and 10
meters per pixel resolution.

Next, you build separate layer for each resolution. In above case, you would
have four layers. Number of needed layers and zoom levels depends on images
quality and on server performance. The best way is to experiment, and see
what is happening.
With method described above, raster layer doesn’t take more then 3 seconds
to load, at any zoom level, and I think this is pretty good result.

All suggestions and comments are welcome.

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